microstrip or patch acts as a resonant λ/2 pareallel plate microstri transmission line with characteristic imp equal to the reciprocal of the number of parallel field cell transmission line. each field cell transmission line has a characteristic imp equal to the intrinsic impedance of the medium
Zi=√µ/ε
and also u can get a basic idea abt microstrip ant in
Hi;
I think that microstrip antenna consists of metallic patches on a microstrip board.
For more exact and complete definition read the Balanis book on antenna.
i can not explain all aspects of microstrip antenna or its full definition overhere but briefly you can think of microstrip antenna as an extension of microstrip transmission line with an abrupt termination
i suggest you to read the book named : "Microstrip antenna design handbook" published by artech house